This spell basically overclocks your damage by a great amouint, it's great for farming and duking out the damage on enemies especially when they are clustered. This is the main reason why the low AP build is possible.

Introduction

Now I know what you're thinking, why make a low AP build when you could be unleashing horrific amount of damage with more AP? Well, I think that's nice, but not necessary. With this build, it focuses more on 3 things: Survivability (Health), Sustainability (Mana and Mana Regen), and Speed (Ability Cooldowns).

Now the reasoning for not leveling Overload until Mid-Late Game is so you can take advantage of its low mana cost for early game harassing, especially because as you're leveling and getting items, Overload still gets bonus damage from your mana capacity. As for maxing Spell Flux first, the reasoning is so that you can use it for last hitting multiple creeps for farming and dealing maximum damage when chasing a hero or fleeing from them. Nothing spells hot death like 5 hits from a single Spell Flux.

Items

Beginning Build Sapphire Crystal Health Potionx2
These are your first items, the Crystal gives you a little extra mana to help with spamming Spell Flux and boosting Overload. As for the Health Pots, they are to help you stay in your lane for a bit longer.

Early Game Tear of the Goddess Mercury's Treads
Now in the early game you want to build towards these two items. The Tear of Goddess will help you get your mana up to help Overload and so you spam your abilities more often for harassing. The Mercury Treads will give you the resistance for snares, stuns, etc. to help you get away from the fight, remember you're squishy.

Mid Game Archangel's Staff Banshee's Veil or Rod of Ages
Mid game, you should have these items, Archangel's Staff continues the basis of the Tear of Goddess, building your mana storage and it offers a little bit of AP which isn't bad. The Banshee's Veil helps you with staying alive in the fight and giving you little bit more magic resistance. However, if there aren't a lot of magic champions in the fight, its safer to build Rod of Ages for that boost in health, mana, and AP.

Late Game Frozen Heart Void Staff
Building the Frozen Heart gives you that much needed ability cooldown reduction so you can keep spamming your spells, mainly Overload because when you reach about 40% cooldown reduction, Overload has a cooldown of 4.2 seconds! This also gives you some armor to deal with the physical champions. As for the Void Staff, this is important at the end of your build, that 40% magic penetration does wonders for your spells.

Extra Stuff
[item=Zhonya's Ring][item=Rylai's Scepter]
These are items that you can shoot for after you finish the items above, Zhonya's for extra AP, Rylai's for health and snaring enemies.

Summoner Abilities

For Summoner Abilities, it's really up to your preference. Personally, I like using ghost and cleanse to help you get out of sticky situations, or to close the gap between you and your enemy. Ignite and Exhaust is good too, but that doesn't seem to work to well against players who actually know what they are doing. I don't suggest Clarity, its nice for extra mana, but overall, you really don't need it at all.

Tips

Early game you want to last hit creeps as much as possible instead of auto-attacking. Additionally, you want to use your Spell Flux to last hit multiple low health creeps and to constantly harass your enemies. Try to not get too close to the fight, you really don't have the health to initiate fights, let alone finish them until you have your ultimate. When you do have your ultimate, make sure you are always using it for team ganks and when your alone, for farming.

My favorite lineup of spells is to use Overload>Desperate Power>Spell Flux>Rune Prison, the reason for casting Overload first is so you can take advantage of your passive, Arcane Mastery. By casting that first, your Overload will be done with its cooldown by the time you go through the whole lineup of spells.